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SHOCK SENSITIVITY OF ENERGETIC MATERIAL AND NANOMETRIC
DAMAGE MECHANISMS IN SILICA GLASS
by
Ken-ichi Nomura
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICS)
May 2008
Copyright 2008 Ken-ichi Nomura

This dissertation focuses on molecular dynamics (MD) simulations of shock sensitivity of energetic material (EM) and nanometric damage mechanisms in amorphous SiO2 (a-SiO2). A scalable parallel MD algorithm incorporating first principles-based reactive force fields (ReaxFF) has been implemented to perform multimillion-to-billion atom chemically reactive MD simulations.; Mechanical stimuli in energetic materials initiate chemical reactions at shock fronts prior to detonation. Multimillion-atom ReaxFF-MD simulations are performed to investigate atomistic mechanisms of shock-induced reaction initiation in 1,3,5-trinitro-1,3,5-triazine (RDX) crystal. The simulation reveals a bi-modal molecular response against a planar shock loading, which creates a nanoscale layered-structure of molecular dipole behind a shock front.; The sensitivity of energetic crystals changes with defects such as voids, grain boundaries and cracks. By performing million-atom ReaxFF-MD simulations, the effects of microstructures in crystal on shock sensitivity have been investigated. MD simulation reveals the formation of a nanojet which focuses into a narrow beam as the void collapses. By increasing particle velocity, a pinning-depinning transition of the shock wave front at the void occurs. Shock loading simulation in a nanophase RDX crystal reveals a deformation mechanism that is mediated by molecular reorientation and conformation changes. Molecular rotation and deformation significantly reduce the energy barrier for the onset of slip.; In contrast to crystalline solids, damage, flow and fracture in glass are still controversial areas. We have performed multimillion-atom MD simulations to investigate initiation and growth of wing cracks in confined silica glass. Under dynamic compression, frictional sliding of precrack surfaces nucleates nanovoids which evolve into nanocrack columns at the precrack tip. Nanocrack columns merge to form a wing crack, which grows via coalescence with nanovoids in the direction of maximum compression.; Multimillion-to-billion-atom MD simulations are performed on nanovoid array under hydrostatic tension. Nanocavities nucleate in intervoid ligaments as a result of the expansion of Si-O rings due to a bond-switching mechanism, which involves bond breaking between Si-O and bond formation between that Si and a nonbridging O. With further increase in strain, nanocracks form on void surfaces and ligaments fracture through the growth and coalescence of ligament nanocavities.

SHOCK SENSITIVITY OF ENERGETIC MATERIAL AND NANOMETRIC
DAMAGE MECHANISMS IN SILICA GLASS
by
Ken-ichi Nomura
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICS)
May 2008
Copyright 2008 Ken-ichi Nomura